IEEE C37.121-1989 pdf free.American National Standard for Switchgear - Unit Substations - Requirements. This standard covers the requirements for three-phase unit substations for step-down operation in the range of 112.5 kVA through 10 (XX) kVA at primary voltages of 601 volts through 38 kilovohs. 1.2 IEEE C37.121 is intended for use as the basis for the coordination of equipment by...